The nuclear power industry long has been engaged in a multitude of studies and investigations seeking improvement in the stamina and reliability of the materials and components forming a reactor-based power system. One such investigation has been concerned with intergranular stress corrosion cracking which heretofore principally has been manifested in the water recirculation piping systems external to the radiation intense reactor core regions of nuclear facilities. Typically, the piping architecture of these external systems is formed of a stainless steel material.
Generally, the studies referred to above have determined that three factors must occur in coincidence to create conditions that promote intergranular stress corrosion cracking. One factor is a sensitization of the metal such as stainless steel, for example, by chromium depletion at grain boundaries. Chromium depletion at grain boundaries may be caused by heat treatment in the course of normal processing of the metal or by welding and like procedures. A second factor is the presence of tensile stress in the material. A third factor is the oxygenated normal water chemistry environment typically present in a boiling water reactor. This latter environment is occasioned by any of a variety of oxidizing and corrosive species contributed by impurities in reactor coolant water. An electrochemical potential monitoring approach has been combined with controlled additions of hydrogen into the coolant to monitor and control the oxygenated environment factor.
Electrochemical potential monitoring is carried out employing paired electrochemical half-cell probes or electrodes which are mounted within the recirculation piping and accessed to the external environment through gland type mountings or the like. Where, as in the instant application, the electrode system of interest involves a metal-metal ion couple, then the reference electrode can conveniently be a metal-metal insoluble salt-anion electrode. A suitable reference electrode may be based, for example, on the half-cell reaction between silver and silver chloride. Calibration of the cell defining electrode pair is carried out by appropriate Nernst based electrochemical calculations, as well as by thermodynamic evaluation in combination with laboratory testing within a simulated environment against a standard electrode.
Half-cell electrodes capable of operation in high pressure and high temperature fluids have been developed for for use in reactor recirculation piping. For example, see U.S. Pat. No. 4,576,667. Such reference electrodes have combined metal housings, ceramic members, and polymeric sealing means formed, for example, from polytetrafluoroethylene or Teflon synthetic resin polymers, to provide electrical isolation of a silver electrode within the reference electrode. These structures have performed adequately in the more benign and essentially radiation free environments of, for example, recirculation piping in nuclear reactors.
Over the recent past, investigators have sought to expand the reference electrode monitoring procedures to the severe environment of the fluid in the vicinity of the reactor core itself for the purpose of studying and quantifying the effect of corrosive species on stress corrosion cracking. Within the reactor core, reference electrodes can be mounted in specially designed small cross section tubing. Such tubing is located among the fuel elements in the reactor core, and is used to house various monitoring devices, such as neutron detectors. As a result, these tubes are known as local power-range monitor tubes.
Thus, the reference electrodes are located in the severe environment of the fluid in the reactor core having a typical high temperature of 274.degree. C., pressure of 1,000 psi, and radiation of 10.sup.9 rads per hour gamma and 10.sup.13 rads per hour neutron. Reference electrode structures of earlier designs are completely inadequate for this reactor core environment, both from a material standpoint, and with respect to the critical need to prevent leakage of radio-active materials to the ambient environment of the reactor. For example, the polymeric seals used in reference electrodes cannot withstand intense radiation with the result being failure of the electrode and leakage of radioactive materials. In known reference electrodes, leakage at the polymeric seal can cause failure due to electrical shorting between a lead wire within the electrode, and the test environment, i.e., the reactor coolant. An electrode which does not suffer from these deficiencies of prior electrodes would therefore be desirable.